Pass upstream client's user agent through to registry on image pulls

Changes how the Engine interacts with Registry servers on image pull.
Previously, Engine sent a User-Agent string to the Registry server
that included only the Engine's version information.  This commit
appends to that string the fields from the User-Agent sent by the
client (e.g., Compose) of the Engine.  This allows Registry server
operators to understand what tools are actually generating pulls on
their registries.

Signed-off-by: Mike Goelzer <mgoelzer@docker.com>